Cordless telephone systems are well known, and digitally communicating cordless telephones have been proposed, one such proposal and its implementation being described in U.K. patent Application No. 8907982, having a publication no. 2,218,548. In the arrangement described, adaptive differential pulse code modulation (ADPCM) is employed for bi-directional digital communication between a cordless telephone and a network station. Each transmitter includes an ADPCM encoder, which receives pulse code modulation (PCM) signals, from an analog to digital converter sampling the output of a microphone providing a speech signal to be transmitted, for example, and a formatting arrangement to add appropriate timing and control data to ADPCM samples for transmission. Each receiver includes an ADPCM decoder to provide a PCM output then received ADPCM data for application to a digital to analog converter and an arrangement responsive to received timing and control data to ensure received ADPCM samples for decoding are correctly identified.
As will be appreciated, the coding algorithms which could be used between receiver and transmitter are many and various; however, it is desirable that communication is in accordance with a standard system so that cordless telephones and network stations are inter-operable. One such standard system which has been widely adopted is known in the art as the Cordless Telephone II (CT2), Common Air Interface (CAI), which it is customary to abbreviate to CT2(CAI). Such standards specify, for example, sampling frequency, sample quantizing level, and formatting and control algorithms. CT2(CAI), for example, specifies sampling at 8 KHz, quantizing to 15 levels, and that cyclic redundancy check data be transmitted as part of the added control data so that data transmission errors can be detected and possibly rectified at the receiver.
ADPCM is specified in CT2(CAI) to reduce the required bit rate from 64K bits per second to a more practicable 32K bits per second, and for the most part provides acceptable results. One problem, however, is that the decoder can deliver a squawk when the radio system enters a fade. This is caused by a delay in the detection of prolonged errors by the control circuits; control data transmission is at a relatively low rate and hence the time taken for the control circuits to make the assessment that signal level in the digital data link has fallen below the threshold requirements of the voice coder is significant. The result is that the voice coder gives a loud squawk between entering the fade and the time the data controller recognizes that bad data is being received and takes action to mute the receiving channel. Typically this squawk lasts for a second or so and is noticeably disruptive to normal speech.
Clearly since the root cause of the problem is insufficient data rate in the control channel for the cyclic redundancy check quickly to detect an extended period of errors, a solution exists simply by increasing the rate. This, however, would be a departure from the CT2(CAI) standard and hence is a solution unavailable to the equipment designer. In any case, an increase in control data rate implies a reduction in useful ADPCM data rate, which in itself would be deleterious to communication. The present invention has been made as a result of a search for a solution to the squawk problem without recourse to data rate increase.